This invention relates to a process for the manufacture of photochromic glass. More particularly, this invention relates to an improved process for annealing photochromic optical quality glass.
Photochromic optical quality glass, which undergoes a reversable photochemical reaction upon exposure to electromagnetic radiation resulting in a change in percent transmission along at least a portion of its characteristic transmission curve, is becoming increasingly popular optical glass, particularly for use in lenses. The glass itself can be any suitable optical quality glass to which one or more photochromic compounds are added in an amount sufficient to alter the light transmission curve of the glass upon exposure to electromagnetic radiation. One example of such glass now becoming popular is the photochromic sunglasses which rapidly darken upon exposure to sunlight and quickly lighten when out of the sun. While for purposes of sunglasses a neutral grey photochromic reaction is preferred so as to not tint the field of vision, in principle similar photochromic glasses are used in various applications which darken to a colored lens. Suitable photochromic compounds include but are not limited to silver in combination with halides.
In the production of photochromic glasses, it is generally necessary to carry out a high temperature annealing treatment of the non-photochromic glass composition (hereinafter referred to as untreated glass), which contains all of the components necessary for the photochromicity. In industrial fabrication, the untreated photochromic glass composition is cooled after the batch melting operation down to a temperature below about 500.degree.C. The untreated glass is then heated to the so-called annealing temperature, usually 550.degree.C-650.degree.C, depending on the type of glass. The annealing temperature is maintained for a period of time sufficient to activate the photochromic properties therein, the length of time required generally varying inversely with the annealing temperature. The now photochromic finished glass is then cooled down to room temperature. Data concerning the processes taking place in the glass during the annealing are known in the art and given in the literature, e.g. Bach and Gliemeroth, Glastechn. Ber. 44: 305 (1971) or J. Amer. Ceram. Soc. 54: 528 (1971), the contents of which are incorporated by reference herein. These glasses and, above all, silver halide-containing glasses, e.g. those described in German Pat. Nos. 1,421,838 and 1,596,847 require extremely accurate control of time and temperature during the annealing process in order to prepare homogeneously photochromic products.
The silver halide-containing glasses described in the above references typically contain 0.015-1.9 wt %, preferably 0.2- 0.9 wt % of silver halides present as silver chloride, silver bromide or silver iodide, or combinations thereof. These values are determined analytically and have the following typical compositions (synthetically):
______________________________________ Broad (Wt.-%) Preferred ______________________________________ SiO.sub.2 0-70 12-61 P.sub.2 O.sub.5 1-50 2-12 B.sub.2 O.sub.3 12-70 13-48 Li.sub.2 O 0.7-8 2-7 Na.sub.2 O 3-15 3-14 U.sub.2 O 0-6 2-4 MgO 2-16 3-10 ZnO 0-20 2-8 BaO 0-20 1-3 PbO 1-60 3-38 La.sub.2 O.sub.3 0-25 2-13 ZrO.sub.2 1-10 2-6 Al.sub.2 O.sub.3 2-25 8-17 ______________________________________
Metal oxides (other than above), 0-15 wt. -%; Ag.sub.2 O, 0.01-2.5 wt. -%; F, 0-10 wt. -%; C1, Br, I, &gt;0.01 wt. -%.
The temperature control problem during annealing has so far been solved by introducing the glass cooled to below 500.degree.C into a furnace, which is heated as precisely as possible by electrical means or by gas, the glass either traveling continuously through a transfer furnace, or being removed at the appropriate time from a chamber furnace; in every case the glass leaves the controlled high temperature zone after an annealing time which is as far as possible within closely defined limits. Variances of about 5.degree.C or 4% of the annealing time cause photochromic defects or inhomogeneities. Thus, two specimens of the same untreated glass, after tempering for 40 minutes at 638.degree.C and 643.degree.C, respectively, show color differences in the activated photochromic condition which can be clearly detected with the naked eye and which are so strong that these two specimen glasses cannot be used simultaneously as photochromic spectacle glasses in a single frame. Moreover, the photochormic properties of these two specimen glasses are different from one another: after being exposed to Xenon light XBO, 250 W, spacing 25 cm, the specimen tempered at 643.degree.C has a light transmissivity lower by 3% at 545 nm than the specimen tempered at 638.degree.C. The speed of regeneration of the specimen tempered at lower temperature is 2 minutes faster, related to the half life period.
However, it has heretofore not been possible to develop an alternative process for the temperature treatment of such untreated glasses without also damaging the quality of the photochromic properties.
One particular disadvantage of the temperature control processes of the prior art is seen in the fact that the "annealing temperature t.sub.a " as used within this specification and being within the softening temperature range, lies considerably above the glass transformation temperature t.sub.g which is similar to the strain point temperature at 10.sup.14.5 poises of the photochromic glass, so that the glass is easily deformed as a plastic during the annealing process. An annealing operation could consequently heretofore be effected only by laying the glass on supports made of metal, kaolin, fire clay or similar high melting materials, the shape of support being assumed by the photochromic glass because of its own weight and plastic state at annealing temperatures.
An additional commercial disadvantage of the prior annealing processes lies in the time required for the annealing, which exceeds by several times the time required for the rest of the manufacturing process.